Level 1 = Could survive with government assistance (basically, just food stamps). Not so much a goal of actual FI, but more of "ok, we can relax just a wee bit now."
Level 2 = Could survive without government assistance.
Level 3 = Can live a little now, occasional meal out, nice vacation once a year.
Level 4 = Live our current lifestyle, except without having work get in the way.
Should hit Level 1 in the next 3-4 years. Then we could slow down a bit and focus on other things (like buying another house). Even with reduced monies going to retirement, compounding interest will continue propelling us forward toward Level 2. I predict that we'll hit Level 4 within 15 years. If my rough numbers are right, and compounding interest will double existing funds approximately every 12 years; then if we just hit it hard and get to Level 1 then become total slackers, we'll still hit Level 4 in another 24 years (each level is a quarter, so Level 1 is 1/4 of Level 4, Level 2 is 2/4, Level 3 is 3/4, and Level 4 is 4/4); because Level 1 will take 12 years to double into Level 2, then another 12 years to double into Level 4. Of course, I don't plan on completely slacking off, but it is nice to know that once you hit a certain level, it'd be really hard to completely screw it up.